Molecular-Level Insight into the Hydroxylated Monomeric VOx/θ-Al2O3(010) and Its Adsorption of Methanol
journal contributionposted on 2019-10-31, 17:03 authored by Wei Lin, Weiqiang Wu, Eric Weitz, George C. Schatz
Unraveling the catalytic functionality of hydrophilic surfaces from a molecular level is of a great challenge, but it offers fundamental guidelines for mechanistic understanding of the heterogeneous catalysis and eventually facilitates the development of highly efficient catalysts. Here, we present a combined experimental and theoretical study of hydroxylated monomeric vanadia species deposited on the θ-Al2O3(010) surface and the subsequent dissociative adsorption of methanol. First, the structure of a hydroxylated θ-Al2O3(010) surface with one monolayer of water coverage is generated. Energies associated with the dissociative adsorption of water indicate that hydroxylation is a strongly exothermic process. The normalized intensities of the vibrational modes of the in situ IR spectra of θ-Al2O3 as a function of temperature show that the hydrated θ-Al2O3(010) surface gradually dehydrates with increase in temperature up to 300 °C, which agrees well with a previous theoretical calculation and shows the presence of strongly adsorbed surface −OH and water. Second, the hydroxylated monomeric VOx/θ-Al2O3(010) species with tridentate VOx structures were investigated for the two most stable adsorption geometries, where three V–O–Al interface bonds form between VOx and the hydroxylated surface. The calculated frequencies of the VO and V–O modes for these two structures agree well with our previous Raman spectra. Finally, dissociative adsorption of methanol for all possible geometries of the two most stable geometries of VOx/θ-Al2O3(010) shows a large range of adsorption energies, with the most exothermic adsorption being 1.29 eV, where the −V–O–CH3 residue prefers to be dangling toward vacuum. The methoxy group geometries were confirmed by the close matching of the calculated frequencies and measured IR spectra. Overall, our study illuminates details of the lowest energy molecular structures and energies of hydroxylated θ-Al2O3(010), VOx/θ-Al2O3(010), and CH3OH/VOx/θ-Al2O3(010). These species are of great interest because of their importance in reactions involving supported vanadia catalysts, both for interpreting experimental studies, and for future use of the structures determined here in studies of reaction mechanisms.